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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_yap.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
</h2></div></div></div>
<h4>
<a name="boost_yap.rationale.h0"></a>
      <span class="phrase"><a name="boost_yap.rationale.decaying_values_captured_in_yap_expressions"></a></span><a class="link" href="rationale.html#boost_yap.rationale.decaying_values_captured_in_yap_expressions">Decaying
      Values Captured in YAP Expressions</a>
    </h4>
<p>
      The main objective of Boost.YAP is to be an easy-to-use and easy-to-understand
      library for using the expression template programming technique.
    </p>
<p>
      As such, it is very important that the way nodes in a Boost.YAP expression
      tree are represented matches the way nodes in C++ builtin expression are represented.
      This keeps the mental model for how to identify and manipulate parts of expression
      trees consistent across C++ builtin and Boost.YAP trees.
    </p>
<p>
      Though this creates minor difficulties (for instance, Boost.YAP terminals cannot
      contain arrays), the benefit of a consistent programming model is more important.
    </p>
<h4>
<a name="boost_yap.rationale.h1"></a>
      <span class="phrase"><a name="boost_yap.rationale.reference_expressions"></a></span><a class="link" href="rationale.html#boost_yap.rationale.reference_expressions">Reference
      Expressions</a>
    </h4>
<p>
      Boost.YAP expressions can be used as subexpressions to build larger expressions.
      <a class="link" href="../"><code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">expr_ref</span></code></a>
      exists because we want to be able to do this without incurring unnecessay copies
      or moves. Consider <code class="computeroutput"><span class="identifier">v2</span></code> and
      <code class="computeroutput"><span class="identifier">v3</span></code> in this snippet from the
      Lazy Vector example. Each is a terminal that owns its value, rather than referring
      to it.
    </p>
<pre class="programlisting"><span class="identifier">lazy_vector</span> <span class="identifier">v2</span><span class="special">{{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">4</span><span class="special">,</span> <span class="number">2.0</span><span class="special">)}};</span>
<span class="identifier">lazy_vector</span> <span class="identifier">v3</span><span class="special">{{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">4</span><span class="special">,</span> <span class="number">3.0</span><span class="special">)}};</span>
</pre>
<p>
      Now consider this expression:
    </p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">v2</span> <span class="special">+</span> <span class="identifier">v3</span><span class="special">)[</span><span class="number">2</span><span class="special">];</span>
</pre>
<p>
      Without using reference semantics, how can we capture this expression, even
      before evaluating it, without copying or moving the vectors? We cannot. We
      must take references to the <code class="computeroutput"><span class="identifier">v2</span></code>
      and <code class="computeroutput"><span class="identifier">v3</span></code> subexpressions to avoid
      copying or moving.
    </p>
<p>
      This comes at a cost. Dealing with <a class="link" href="../"><code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">expr_ref</span></code></a> expressions complicates user
      code. The alternatives, silently incurring copies/moves or disallowing the
      use of subexpressions to build larger expressions, are worse.
    </p>
</div>
<div class="copyright-footer">Copyright © 2018 T. Zachary Laine<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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